Method and apparatus for polishing inner an internal surface of an aluminum extrusion hollow shape

ABSTRACT

There is provided an electrolytic integrated polishing apparatus which enables high precision polishing of the internal surface of a long sized cylindrical workpiece such as a metallic tube. The apparatus includes a work supporting unit  13  for disposing the cylindrical workpiece W, so that the axial center of the cylindrical portion is aligned with the vertical direction, a rotation shaft  4  supported downward along the vertical direction and free-rotatably supported in an external tube  3  which is freely movable along the vertical direction, a tool electrode  5  including a grindstone directed to radial directions, attached to the tip of the rotation shaft  4,  and a plastic tube  6  wound around the external surface of the external tube  3  for pressurization.

FIELD OF THE INVENTION

The present invention relates to a polishing method and apparatus formirror processing of the internal surface of a long sized cylindricalworkpiece such as a metallic tube, shape or the like, including analuminum extrusion hollow shape with a cylindrical portion, usingelectrolytic integrated polishing technology, and to a long sizedcylindrical workpiece such as an aluminum extrusion hollow shapepolished by electrolytic integrated polishing according to the method.

BACKGROUND ART

Cylindrical portions of fluid pressure cylinders such as hydrauliccylinders, air cylinders or the like are required to have roundness of25 μm for hydraulic cylinders and 150 μm for air cylinders. For example,because aluminum extrusion hollow shapes used for these cylinders haveinadequate precision by normal extrusion only, roundness or dimensionalprecision of cylindrical portions of most products thereof are improvedby machining such as cutting or by post-machining burnishing. However, amaterial, the cylindrical portion of which has a small bore or alongitudinal size larger than the bore, can not be machined. Therefore,the current situation has been such that, for example, yield isdeclining for not achieving required roundness, or productivity isstaying low for having to machine the pieces, one by one, of a longsized material which must be cut into product length beforehand.

On the other hand, electrolytic integrated polishing technology, ahybrid of elution by electrolyte and abrasion by abrasives, is known asa method for polishing the surface of metal with a high precision. Theelectrolytic integrated polishing technology, applied to mirrorfinishing of the internal surface of metallic tubes, generally inserts arotation shaft, to the top of which a tool electrode is attached, into ametallic tube, rotates the metallic tube about the rotation shaft, thensupplies electrolyte inside the metallic tube and passes current, andpolishes the internal surface of the metallic tube by a grindstoneattached to the tool electrode, withdrawing the rotation shaft slowly.

With such electrolytic integrated polishing methods for the internalsurface of metallic tubes, abrasion of the grindstone prevents highprecision mirror finishing due to inappropriate polishing of theinternal surface. Therefore, apparatuses coping with abrasion of thegrindstone by applying pressing force onto the grindstone via a leaf, adiaphragm or the like are conventionally known. (See Japanese UtilityModel Laid-Open Publication No. Hei 4-130120, 5-86429 and so on.)

The above mentioned conventional method using a leaf can not preservethe desired polishing performance because pressing force decreases asthe grindstone abrades, especially in narrow places such as the internalsurface of a long sized metallic tube. Besides, there have beenshortcomings such as the necessity of a specific insertion guide forinserting the polishing tool into the metallic tube because the leafgenerates force by being bent to a predetermined extent.

On the other hand, although a method using a diaphragm can resolve theabove mentioned problem caused by the leaf, a plurality of diaphragmscorresponding to a plurality of grindstones must be individuallydisposed, which makes processing of the installation sections, andinstallation work complicated. Besides pressing force applied toindividual grindstones is not uniform due to variation occurred in theplurality of diaphragms, which makes mirror finishing with a highprecision impossible.

The above mentioned method is mainly applied to processing of theinternal surface of pipes made from steel or stainless steel. Mostapparatuses are horizontal types that can easily perform processing oflong sized products, while there are few vertical types. (See JapanesePatent Laid-Open Publication No. Hei 3-98758.) Furthermore, there is noconcrete example of application to aluminum extrusion shapes having avariety of external shapes.

Machining (cutting, for example) of the internal surface of thecylindrical portion of an aluminum extrusion hollow shape is moredifficult than processing of external surface. Besides, long materials,with difficulties in processing due to problems with rigidity of thetool, must be cut short for processing in order to preserve a certainprocessing precision. Therefore, other than increase in cost, ultra highprecision mirror finishing of aluminum alloy by cutting has beendifficult because aluminum alloy, having a low rigidity which is aboutone-third of steel and a coefficient of thermal expansion twice as largeas steel, is easy to be deformed by cutting resistance or cutting heat.

On the other hand, electrolytic integrated polishing which is an ultrahigh precision finishing technology with a roundness of finished surfaceequal to or smaller than 10 μm and a surface roughness equal to orsmaller than 1 μm, can be applied to long sized pipes. However, aproblem remains in that directly applying electrolytic integratedpolishing methods and apparatuses that have been mainly used forpolishing of the internal surface of steel or stainless steel tubes topolishing of the internal surface of a cylindrical portion of analuminum extrusion hollow shape will not result in processing with ahigh precision.

For example, the inventors of the present invention failed to obtain thedesired processing precision by electrolytic integrated polishing of theinternal surface of an aluminum extrusion tube using a conventionalelectrolytic integrated polishing apparatus (a type in which axialdirections of a long sized metallic tube and a tool electrode arehorizontally oriented, and the metallic tube and the tool electrode arerotated in opposite directions with each other.) This is becauseprocessing needed be performed with the pressing force of the grindstonecontrolled at a low pressure due to softness and easiness to deformationof aluminum, having a low strength and a low rigidity which are aboutone-third of steel or stainless steel, and because precision ofprocessing degrades by deflection of the axial center due to influenceof weight of the tool electrode supported horizontally by the rotationaxis.

Besides, since rotating metallic shapes having a variety of externalshapes is virtually impossible, long sized cylindrical workpieces towhich the above mentioned electrolytic integrated polishing apparatuscan be applied are limited to cylinder pipes.

It is an object of the present invention, having been made consideringthe above mentioned problems with prior arts, to provide an electrolyticintegrated polishing method and apparatus which enables high precisionpolishing of the internal surfaces of the cylindrical portions ofmetallic shapes having a variety of external shapes such as aluminumextrusion hollow shapes or the like, and to provide long sizedcylindrical workpieces such as aluminum extrusion hollow shapes havinginternal surfaces of the cylindrical portions polished with a highprecision.

DISCLOSURE OF THE INVENTION

The method according to the present invention is an electrolyticintegrated polishing method for polishing the internal surface of thecylindrical portion of a long sized cylindrical workpiece such as analuminum extrusion hollow shape by integrating elution by electrolyteand abrasion by a grindstone attached to a tool electrode inserted intothe cylindrical portion, characterized by disposing the long sizedcylindrical workpiece so that the axial center of its cylindricalportion is aligned with the vertical direction, inserting the toolelectrode attached to the tip of a rotation shaft supported downwardsimilarly along the vertical direction into the cylindrical portion, androtating as well as relatively moving the tool electrode vertically.

Besides, the apparatus according to the present invention is anelectrolytic integrated polishing apparatus for polishing the internalsurface of the cylindrical portion of a long sized cylindrical workpiecesuch as an aluminum extrusion hollow shape by integrating elution byelectrolyte and abrasion by a grindstone attached to a tool electrodeinserted into the cylindrical portion, characterized by comprising awork supporting unit for disposing the long sized cylindrical workpieceso that the axial center of its cylindrical portion is aligned with thevertical direction, a rotation shaft supported downward along thevertical direction and inserted into the cylindrical portion of theabove mentioned long sized cylindrical workpiece, the tool electrodeattached to the tip of the rotation shaft, and a transportation unit formoving the above mentioned rotation shaft and/or the work supportingunit along the axial direction.

In the present method and apparatus, because the rotation shaft issupported downward along the vertical direction and the tool electrodeis attached to the tip thereof, influence of the weight of the toolelectrode and the rotation shaft themselves is eliminated to suppressdeflection of the tool electrode, which improves the precision ofprocessing. Besides, the present method and apparatus can be similarlyapplied to a variety of metallic shapes such as an aluminum extrusionhollow shape having a variety of external shapes, because only the toolelectrode is rotated without rotating the long sized cylindricalworkpiece.

Here, the above mentioned transportation unit may be disposed at eitherone of or both sides of the rotation shaft and the work supporting unit.That is, any disposition will do if the long sized cylindrical workpieceand the tool electrode move along the axial direction relatively to eachother.

Note that as an embodiment of the above mentioned polishing apparatus,the above mentioned rotation shaft is free-rotatably supported inside anexternal tube supported downward along the vertical direction. Theexternal tube does not rotate, preferably covers almost throughout thewhole length of the rotation shaft except for the tool electrode at thetip thereof, and is moved along the axial direction simultaneously withthe rotation axis. In this case, since the rotation shaft isfree-rotatably supported at the center of the external tube, deflectionof the tool electrode when rapidly rotating is suppressed to improve theprecision of processing.

Besides, when the above external tube is provided, it is preferable towind a plastic tube spirally around the peripheral thereof and to enablepressurization inside the plastic tube. When the external tube isinserted, for example, into the cylindrical portion of a long sizedcylindrical workpiece, such as an aluminum extrusion hollow shape, andpressure is applied onto the plastic tube at the state, the plastic tubeexpands and is pressed against the internal surface of the cylindricalportion. The effect, preventing minute deflections caused by rotation ofthe rotation axis and the tool electrode and maintaining the externaltube always at the center of the cylindrical portion, results in furtherimprovement of the precision of processing.

Furthermore, preferably a hollow portion is provided inside the toolelectrode, a pressure tube composed of silicon tube or the like, insideof which can be pressurized, is provided in the above mentioned hollowportion. By pressurizing inside the pressure tube, elastic grindstonescan be pressed with a constant pressure toward radial directions.Besides, a constant pressing force is always maintained when thegrindstones are worn out.

Moreover, by the electrolytic integrating polishing method andapparatus, it becomes possible to perform finish polishing, withoutmachining, of a long sized metallic shape such as an aluminum extrusionhollow shape, the length of the cylindrical portion of which is tentimes as large as the diameter, or more, with a roundness of theinternal surface of the cylindrical portion equal to or smaller than 10μm and a surface roughness Rmax along the axial and circular directionsequal to or smaller than 1 μm.

Other characteristics of the present invention will be described indetail in the following sections about preferred embodiments of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general view of an electrolytic integrated polishingapparatus according to the present invention.

FIG. 2 is a view illustrating the major portion thereof.

FIG. 3 is a vertical cross sectional view of the tool electrode.

FIG. 4 is a horizontal cross sectional view of the electrode at A—Aplane in FIG. 3.

FIG. 5 is a cross sectional view of an aluminum extrusion hollow shapeused in the example.

BEST MODE FOR CARRYING OUT THE INVENTION

The electrolytic integrated polishing apparatus according to the presentinvention will be described in detail hereinafter with reference toFIGS. 1 to 5.

As shown in FIGS. 1 and 2, the electrolytic integrated polishingapparatus includes a platform 1, a frame 2 disposed on the platform, anexternal tube 3 vertically disposed, a rotation shaft 4 vertically andfree-rotatably disposed via a plurality of bearings (not shown) insidethe external tube 3, a tool electrode 5 attached to the tip of therotation shaft 4 at the lower portion of the external tube 3, a plastictube 6, inside of which can be pressurized by a mechanism (not shown),comprising a silicon tube or the like and being spirally wound aroundthe peripheral of the external tube 3, a guide 7 attached to the frame2, a sliding member 8 freely and vertically slidable along the guide 7,a supporting member 9 fixed to the sliding member 8 for supporting theupper end of the external tube 3, a bearing member 10 fixed to thesliding member 8 for supporting nearby the upper portion of the rotationshaft 4, a transportation motor 11 for sliding the sliding member 8vertically along the guide 7 thus moving the external tube 3 and therotation shaft 4 upward and downward, a motor 12 attached to the slidingmember 8 for rotating the rotation shaft 4, and a fixed chuck 13(positive pole energizing chuck) for fixing a long sized cylindricalworkpiece, such as, for example, an aluminum extrusion hollow shape W.

In the electrolytic integrated polishing apparatus, the rotation shaft 4is a hollow shaft (see FIG. 3) the top of which enabling introduction ofair into a hollow bore 4 a via a rotary joint 15. Besides, the presentapparatus also includes a free ring mechanism 16, which is located abovethe fixed chuck 13, freely slidable along the guide 7 and can be fixedto any appropriate position, a guide sleeve 17 disposed on the free ringmechanism 16, a liquid receiver 18 for receiving electrolyte flowing outfrom an opening of the guide sleeve 17, a free ring mechanism 19 fixedat a lower position of the fixed chuck 13, and a negative poleenergizing brush 20 contacting the rotation shaft 4.

The free ring mechanisms 16 and 19 have approximately the samestructure, wherein free rings 23, 24, 25 and 26 are free-rotatablysupported, in two layers within fixed sleeves 21 and 22, independent ofeach other, via a mechanical seal. Then, the free ring mechanisms 16 and19 are kept in contact, via seal packings 27 and 28 with the upper andthe lower openings of the cylindrical portion of the aluminum extrusionhollow shape W attached to the fixed chuck 13. The free rings 23 to 26have the same or slightly larger bore than the finished bore of thecylindrical portion of the aluminum extrusion hollow shape W, andapproximately the same length as that of an elastic grindstone,described hereafter, of the tool electrode 5. The free rings 23 to 26are rotated freely and synchronously by the pressing force of theelastic grindstone, and prevent the end surface of the cylindricalportion of the aluminum extrusion hollow shape W from deforming into abell-mouth shape because of the polishing.

Besides, the guide sleeve 17, successively disposed on the free ringmechanism 16, has approximately the same bore as those of the free rings23 and 24.

As shown in FIG. 3, the rotation shaft 4 is free-rotatably supported inthe external tube 3 by bearing 30. Besides, a felt-like seal 33intermediates between a seal holder 31 screwed into the lower end of theexternal tube 3 and a seal holder 32 fixed to the rotation shaft 4 toprevent electrolyte from flowing in between the external tube 3 and therotation shaft 4 which are rotating relatively to each other.

The tool electrode 5 is attached, via a mounting member 34, to the tipof the rotation shaft 4 projecting from the lower end of the externaltube 3. As also shown in FIG. 4, hollow electrodes 35 and 36 areattached to the upper and the lower sections of the tool electrode 5,apart from each other by a predetermined interval. Openings are providedto each of the electrodes 35 and 36, in a radial manner, 180 degreesapart from each other. The openings of each of the electrodes 35 and 36are faced to directions 90 degrees apart. In each opening, as abrasives,elastic grindstones 37 and 38 for coarse polishing and finishing,respectively, are supported as well as a holder 39 along radialdirections to be freely slidable. Disposed in the hollow of the toolelectrode 5 is a pressure tube 41, composed of a silicon tube or thelike, one end of which communicates with the hollow bore 4 a of therotation shaft 4 while the other end thereof is sealed by a plug 40. Thepressure tube 41 expands by air introduced into the hollow bore 4 a viathe rotary joint 15, and presses the elastic grindstones 37 and 38,toward radial directions, against the internal surface of thecylindrical portion of the aluminum extrusion hollow shape W with aconstant pressure according to the air pressure. A hard glass 31 isattached to the side walls of the openings of the electrodes 35 and 36to reduce the friction between the electrodes and the holder 39.

In order to prevent leakage current, insulation is provided between thesupporting member 9 and the external tube 3 and between the bearingmember 10 and the rotation shaft 4, and the external surface of theexternal tube 3 is insulated by sheathing. Besides, the metallicportions of the tool electrode 5 are insulated by sheathing except thatthe sides of the external surface of the electrode 35 sandwiching theelastic grindstone 37 are exposed. As for the exposed surface, the frontside of the direction of rotation of the elastic grindstone 37 is anexposed surface 35 a for passivation coating generation, and the rearside thereof is an exposed surface 35 b for intensive elution.

The plastic tube 6 wound around the external tube 3 is, as shown in FIG.2, located between the external surface of the external tube 3 and theinternal surface of the cylindrical portion of the aluminum extrusionhollow shape W, the free rings 23 and 24, and the guide sleeve 17,respectively. Applying pressure inside of the plastic tube 6 expands andpresses the plastic tube 6 against the internal surface of the abovementioned elements, which prevents minute deflections of the externaltube 3 caused by rapid rotation of the rotation shaft 4 and the toolelectrode 5.

The polishing process using the electrolytic integrated polishingapparatus will be described next.

Firstly, the aluminum extrusion hollow shape W to be processed isgrasped by the fixed chuck 13 and fixed on the free ring mechanism 19via the seal packing 28, and the free ring mechanism 16 is fixed on thealuminum extrusion hollow shape W via the seal packing 27. Here, theaxial center of the internal surfaces of the cylindrical portion of thealuminum extrusion hollow shape W, each of the free rings 23 to 26 andthe guide sleeve 17 must be aligned on the same axial line, and, at thesame time, they must be aligned on the same axial line of the externaltube 3, the rotation shaft 4 and the tool electrode 5. (see FIG. 1).

Under this situation the transportation motor 11 is driven to bring downthe sliding member 8 and to insert the tool electrode 5 from the guidesleeve 17. Thus the bringing down is stopped at a position shown in FIG.2, that is, a position where the elastic grindstone 37 for coarsepolishing meets the lower position, for example, being the location ofthe free ring mechanism 19, of the cylindrical portion of the aluminumextrusion hollow shape W. Because no air is introduced into the pressuretube 41 and the inside of the plastic tube 6 is not pressurized, thetool electrode 5 and the external tube 3 can be smoothly insertedwithout resistance.

Next, air is introduced into the plastic tube 6 and the pressure tube 41for pressurization, and electrolyte is introduced from the electrolyteinjection opening at the lower portion of the free ring mechanism 19.Then the motor 12 is driven to rapidly rotate the rotation shaft 4 andthe tool electrode 5. The rotation shaft 4 and the fixed chuck 13 areenergized, setting the electrodes 35 and 36 to be the negative pole andthe aluminum extrusion hollow shape W to be the positive pole anddriving the transportation motor 11 to bring up the tool electrode 5 ata constant velocity.

Now, the electrolyte rises through the gap between the tool electrode 5and the internal surfaces of the free rings 25 and 26, and passesthrough the gap between the internal surface of the cylindrical portionof the aluminum extrusion hollow shape W and the external surface of theexternal tube 3, the gap between the internal surface of the free rings24 and 23 and the external surface of the external tube 3, and the gapbetween the internal surface of the guide sleeve 17 and the externalsurface of the external tube 3. Then the electrolyte is discharged tothe liquid receiver 18, recovered and, after having the polishing sludgeseparated by precipitation and filtered, forcedly fed again to theelectrolyte injection opening.

At the beginning of the polishing process, the elastic grindstones 37and 38 are pressed against the internal surfaces of the free rings 25and 26, however the free rings 25 and 26 will not be polished becausethey are rotating in synchronization with the rotation of the elasticgrindstones 37 and 38. Here, by disposing the free rings 25 and 26 intwo layers with approximately the same length as the elastic grindstones37 and 38, inertia weight of each becomes small, which results in betterresponse to the rotation of the elastic grindstones 37 and 38.

As the tool electrode 5 rises and the elastic grindstone 37 comes intothe cylindrical portion of the aluminum extrusion hollow shape W, theelastic grindstone 37 is pressed against its internal surface andpolishing is performed based on the following principle. That is, whilethe electrode 35 is rapidly rotating, a thin passivation coating isgenerated on the internal surface of the cylindrical portion via theexposed surface 35 a for passivation coating generation before theelastic grindstone 37 begins polishing. Then the elastic grindstone 37abrades the internal surface of the cylindrical portion, results inremoving the passivation coating which lacks viscosity and exposing themetallic base. Immediately after that, electrolytic current concentrateson the height of the metallic base via the exposed surface 35 b forintensive elution and performs selective electrolysis. Following theabove, the tool electrode 5 gradually rises and the electrolyticintegrated polishing is performed on the internal surface of thecylindrical portion until the elastic grindstones 37 and 38 arewithdrawn from the cylindrical portion of the aluminum extrusion hollowshape W, smoothing the internal surface.

At the initial stage of the polishing process, the free rings 25 and 26prevent the end surface of the cylindrical portion of the aluminumextrusion hollow shape W from being polished into a bell-mouth shape.That is, with regard to the elastic grindstone 37 for example, on itsway entering the cylindrical portion, a portion of the elasticgrindstone 37 is pressed against the internal surface of the cylindricalportion while the rest of the portions thereof are pressed against thefree ring 25. However, because the bores of the cylindrical portion andthe free ring 25 are the same, the pressed surface of the elasticgrindstone 37 being pressed does not tilt, therefore the end surface ofthe cylindrical portion will not be polished into a bell-mouth shape.

When the elastic grindstones 37 or 38 are being withdrawn from thecylindrical portion of the aluminum extrusion hollow shape W at thefinal stage of the polishing process, the free ring 24 prevent the endsurface of the cylindrical portion from being polished into a bell-mouthshape.

While the polishing process is being performed, the plastic tube 6 ispressurized and pressed against the internal surfaces of the cylindricalportion of the aluminum extrusion hollow shape W, the free rings 23 and24, and the guide sleeve 17, respectively preventing minute deflectionsof the external tube 3 caused by rapid rotation of the rotation shaft 4and the tool electrode 5, which eventually prevents deflection of thetool electrode 5 in the cylindrical portion. By disposing the guidesleeve 17 with a constant length successively above the free ringmechanism 16, the plastic tube 6 can provide a contacting internalsurface until the elastic grindstones 37 and 38 are completely withdrawnfrom the cylindrical portion of the aluminum extrusion hollow shape Wand stopped rotating, which can prevent deflection of the external tube3.

Here, metallic tubes or shapes made from steel, stainless steel,aluminum, aluminum alloy or the like may be exemplified, although notlimited to them, for the long sized cylindrical workpiece to which themethod according to the present invention is applied. Besides, thelength of the cylindrical portion is ten times as large as the diameter,or more. The long sized cylindrical workpiece is finished to have aroundness of the internal surface of the cylindrical portion thereofequal to or smaller than 10 μm and a surface roughness Rmax equal to orsmaller than 1 μm, by the electrolytic integrated polishing.

EXAMPLE 1

A 600 mm long aluminum extrusion hollow shape, having a cross sectionshown in FIG. 5 was disposed in a vertical type electrolytic integratedpolishing apparatus shown in FIG. 1 for polishing its centralcylindrical portion (32 mm of finishing bore). Polishing was performedwith the tool electrode having two-layered electrodes and elasticgrindstones for coarse polishing and finishing, and under a conditionthat the electrolyte was sodium nitrate aqueous solution (20%), appliedvoltage was 8 V, rotation speed of the tool electrode was 2.5 m, risingspeed was 1 m per minute.

As a result, roundness of the shape at the central cylindrical portion,which was 450 μm before polishing, became 9.2 μm after polishing,surface roughness (Rmax) along axial direction, which was 0.8 μm beforepolishing, became 0.5 μm after polishing, and surface roughness (Rmax)along circular direction, which was 1.98 μm before polishing, became 0.3μm after polishing, all of which showed a largely improved precision.

EXAMPLE 2

A 600 mm long stainless steel cold-finished tube was disposed in avertical type electrolytic integrated polishing apparatus shown in FIG.1 for polishing its internal surface (32 mm of finishing bore).Polishing was performed with the tool electrode having two-layeredelectrodes and elastic grindstones for coarse polishing and finishing,and under a condition that the electrolyte was sodium nitrate aqueoussolution (20%), applied voltage was 8 V, rotation speed of the toolelectrode was 3.0 m, rising speed was 0.4 m per minute.

As a result, surface roughness (Rmax) of the base tube, which was 10 μmbefore polishing, became after finishing, 0.2 μm with a grain size #1500and 0.08 μm with a grain size #6000, all of which provided an excellentmirror finishing.

APPLICABILITY TO INDUSTRY

According to the present invention, polishing of the internal surface ofthe cylindrical portion of a long sized cylindrical workpiece with ahigh precision in terms of roundness and surface roughness becomespossible, without machining. Therefore, for example, a process, whereina workpiece still in the form of a long sized material is polished andafterwards cut into pieces with a length of a fluid pressure cylinder,can be performed, improving the efficiency of the high-precisionpolishing and the productivity. Additionally, the present invention canbe applied to high-precision polishing of the internal surfaces of thecylindrical portions of metallic shapes having a variety of externalshapes.

1. An electrolytic integrated polishing apparatus for polishing the internal surface of the cylindrical portion of an aluminum extrusion hollow shape by integrating elution by electrolyte and abrasion by a grindstone attached to a tool electrode inserted into an inside of the cylindrical portion, said apparatus comprising a work supporting unit for disposing the aluminum extrusion hollow shape so that an axial center of the cylindrical portion is aligned with a vertical direction, a rotation shaft supported downward along the vertical direction and inserted into the inside of the cylindrical portion of said aluminum extrusion hollow shape, a tool electrode attached to a tip of the rotation shaft, and a transportation unit for moving said rotation shaft and/or the work supporting unit along the axial direction; and wherein a free ring mechanism is disposed at and axially aligned with one or more of an upper and lower openings of the cylindrical portion of the aluminum extrusion hollow shape, said free ring mechanism has a sleeve and a free ring supported free-rotatably within the sleeve, and has approximately the same bore as a finished bore of said cylindrical portion and has approximately the same length as that of an elastic grindstone, and is rotatable freely and synchronously by a pressing force of the elastic grindstone.
 2. The apparatus for polishing the internal surface of the cylindrical portion of an aluminum extrusion hollow shape according to claim 1, further comprising a hollow portion inside the tool electrode, and a pressure tube, which can be pressurized, in said hollow portion. 